This invention relates to systems and methods of graphically demodulating graphical bar codes.
A typical bar code symbol is a pattern of parallel bars and spaces of various widths that represent data elements or characters. The bars represent strings of binary ones and the spaces represent strings of binary zeros. A conventional xe2x80x9cone-dimensionalxe2x80x9d bar code symbol contains a series of bars and spaces that vary only in a single dimension. One-dimensional bar code symbols have relatively small information storage capacities. xe2x80x9cTwo-dimensionalxe2x80x9d bar codes have been developed to meet the increasing need for machine-readable symbols that contain more information than one-dimensional bar code symbols. The information storage capacity of two-dimensional bar code symbols is increased relative to one-dimensional bar codes by varying the bar code patterns in two dimensions. Common two-dimensional bar code standards include PDF417, Code 1, and Maxicode. One-dimensional and two-dimensional bar code symbols typically are read by optical scanning techniques (e.g., by mechanically scanned laser beams or by self-scanning charge-coupled devices (CCD""s)) that convert a printed bar code symbol into electrical signals. The electrical signals are digitized and decoded to recover the data encoded in the printed bar code symbol.
Bar codes may be used in a variety of applications, including low information content applications (e.g., automatic price tagging and inventory management), and relatively high information content applications (e.g., encoding mail addresses and postage for automated mail reading and mail distribution systems, and encoding compressed content of a printed page).
As used herein, the term xe2x80x9cgraphical bar codexe2x80x9d broadly refers to an image that contains inconspicuous graphical modulations that encode embedded information. The invention features systems and methods of graphically demodulating a graphical bar code automatically without foreknowledge of the original unmodulated base image, but rather based upon a base image that is derived based upon intrinsic features of the graphical bar code.
In one aspect, the invention features a method of decoding a graphical bar code corresponding to an original base image that is modulated with a graphical encoding of an original message. In accordance with this inventive method, one or more intrinsic features of the graphical bar code are measured. Based upon the intrinsic feature measurements, a sequence of graphical templates is selected from a predefined template set of information-encoding graphical templates. The graphical bar code is graphically demodulated with selected sequence of graphical templates to produce a message.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The step of selecting the sequence of graphical templates preferably comprises deriving a base image corresponding to the original image based upon the intrinsic feature measurements.
In some embodiments, the step of selecting the sequence of graphical templates comprises selecting a template for each region of the graphical bar code likely to match a corresponding region of the original base image. A weighted average of gray value measurements may be computed over each graphical bar code region. The weighted average of gray values may be computed by fitting a mask to dot locations over the given graphical bar code region. The mask may have a truncated Gaussian profile.
In some of these embodiments, the step of selecting a template for each region of the graphical bar code comprises computing template probabilities for each graphical bar code region. Template probabilities may be computed for a given bar code region based upon the weighted average of gray value measurements over each graphical bar code region. In some of these embodiments, parameters of probability distributions fit to a histogram of the weighted average gray values may be estimated.
In other embodiments, the step of selecting a template for each region of the graphical bar code comprises assigning a dark pixel value or a bright pixel value to each pixel based upon a threshold that is applied to the masked average gray value measurements.
In some embodiments, the step of selecting the sequence of graphical templates comprises selecting a base image that is likely to match the original base image from a set of possible base images. The matching base image may be selected based upon information that is encoded in the graphical bar code. For example, the matching base image may be selected based upon size and aspect ratio characteristics of the graphical bar code. The matching base image may be selected based upon correlation between the graphical bar code and base images of the set of possible base images. The matching base image also may be selected based upon a message that is associated with the graphical bar code. The message may be embedded in a bar code located in a predefined position relative to the graphical bar code, in a marking pattern surrounding the graphical bar code, or in a marking pattern having a different radiant energy response than the graphical bar code.
In another aspect, the invention features a computer program for decoding a graphical bar code corresponding to an original base image that is modulated with a graphical encoding of an original message. The computer program resides on a computer-readable medium and comprises computer-readable instructions for causing a computer to: measure one or more intrinsic features of the graphical bar code; based upon the intrinsic feature measurements, select a sequence of graphical templates from a predefined template set of information-encoding graphical templates; and graphically demodulate the graphical bar code with the derived sequence of graphical templates to produce a message.